mail-envelope-from

mail-envelope-from is a variable defined in `sendmail.el'.
Its value is

nil

Documentation:
If non-nil, designate the envelope-from address when sending mail.
This only has an effect if `mail-specify-envelope-from' is non-nil.
The value should be either a string, or the symbol `header' (in
which case the contents of the "From" header of the message
being sent is used), or nil (in which case the value of
`user-mail-address' is used).

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.